1. Field of the Invention
The invention relates to an apparatus for magnetically recording and reading wide-band signals on a magnetic tape in signal tracks which extend obliquely adjacent each other; and more particularly to such an apparatus comprising an at least partly cylindrical drive spindle with a free end, which spindle is rotatable about an axis of rotation; at least one magnetic head which is rotatable in a circular path around the axis of rotation of the drive spindle for writing and/or reading the oblique signal tracks on the magnetic tape; a head support for the magnetic head(s), which support is detachably secured to a cylindrical portion of the drive spindle near its free end and which is formed with a central bore for mounting the head support on the drive spindle; as well as means for detachably mounting the head support on the drive spindle and comprising centering means for centering the head support relative to the axis of rotation of the drive spindle, and fixing means for rigidly connecting the magnetic head support and the drive spindle to each other.
Apparatus of the afore-mentioned type is employed in so-called helical-scan video recorders for magnetically recording and reading video signals. Such video recorders are mass manufactured for the consumer market and thus are a typical mass product. Among the known consumer equipment for magnetically recording and reading signals they constitute the most advanced type, because very stringent requirements are imposed on the accuracy with which the signals are written on and should subsequently be readable from the magnetic tape by means of the rapidly rotating magnetic heads. The magnetic tape is practically always accommodated in a cassette which is of a specific standard type. An important requirement is that it should be possible that cassettes which have been used for making a recording on one video recorder can be placed on an other similar video recorder for the reproduction of the previously made recording without a perceptible reduction in the quality of reproduction in comparison with a reproduction using the video recorder with which the recording was made originally. In practice this requirement of interchangeability of standard cassettes and of video recorders of a specific type is generally referred to as the compatibility requirement.
To make the most economic use of the magnetic tape employed in the cassettes and thus to raise the playing time attainable with a specific cassette, many attempts have been made to maximize the density of the signals recorded on the magnetic tape with video recorders. In modern video recorders for the consumer market oblique tracks are written on the magnetic tape by means of two magnetic heads, which tracks typically have a width of approximately 40 microns. The distance between consecutive tracks is approximately 30 microns, so that the tracks slightly overlap each other. The length of the oblique track is approximately 100 mm and is consequently very great in comparison with the width of the track. These dimensions for the track width and for the track distance should be maintained within narrow limits over the total length of the oblique track in view of the compatibility requirement.
This means that a very high positional accuracy of the magnetic heads is required relative to those elements of the video recorder which provide guidance of the tape along the head support, so that the small tolerance limits of the tracks on the magnetic tape are achieved as a result of small tolerance ranges in respect of the position of the magnetic heads.
For the most frequently used video recorders, which comprise a plurality of magnetic heads, it is furthermore of importance that the synchronism of the signals read by the magnetic heads is guaranteed within narrow limits. Synchronizing errors may manifest themselves as visible timing errors in the reproduced television picture, giving rise to instabilities at the upper edges of the pictures.
During manufacture the magnetic heads are positioned on the head support with special precision devices, for example using optical measurement, the head support being mounted on a spindle. The height of the magnetic heads relative to the tape guide members of the video recorder should be adjusted with a tolerance of the order of magnitude of 5 microns, in such a way that the mutual difference in height between the magnetic heads should not exceed 5 microns. Moreover, in the case of a head support in the form of a rotary head disc with a diameter of 65 mm the eccentricity of the magnetic heads relative to the axis of rotation of the head support should be less than 11/2 micron. A further requirement which is imposed is that the magnetic heads are uniformly spaced over the circumference of the head support in an accurate manner. For example, when two magnetic heads are used, the lengths of arc between the two magnetic heads should not differ more than 3 microns from each other.
2. Description of the Prior Art
An apparatus of the type described above is for example known from U.S. Pat. No. 3,422,230. In this apparatus a flange is secured to the drive spindle, on which flange a head support can be mounted by means of screws. On its side which faces the head support the flange is accurately machined and the head support is already provided with the magnetic heads, so that at least the distance between the magnetic heads can accurately be adjusted in advance. This is necessary as the compatibility of the video recorder should not be lost in the case of replacement or repair of a head support. Thus, for securing and centering the head support such steps should be taken that after removal of the head support and refitting a new head support, for example for service purposes, compatibility is guaranteed. In view of the very small tolerances which are permissible in respect of the eccentricity of the magnetic heads relative to the drive spindle, precise centering of the head support within the range of the specified 11/2 microns is very important. Yet it is desirable that centering can be effected by a service engineer rapidly and without the use of position-alignment devices. In the known apparatus the head support is centered on the drive spindle by mounting the head support on an annular projection of the previously mentioned flange, which engages in the central bore of the head support with a specific fit. Different embodiments employ a centering member which is interposed between the spindle and the central bore. In view of the tolerances with which spindles and bores can be made in mass production it is extremely difficult if not impossible to provide such very small permissible eccentricity tolerances with these constructions. Therefore, such centering methods are not satisfactory for the latest video recorders.